IMS (IP multimedia Subsystem) is an open architecture for providing IP multimedia services in mobile, fixed, and mobile-fixed convergence environment on the basis of SIP (Session Initiation Protocol) signaling. In the IMS architecture, it is possible to create, control, and modify an application regardless of the type of network or platform and implement the communication of multimedia and other large volume data efficiently. In an exemplary IMS-based communication system, it is possible for the mobile users to participate in a real time group communication supporting one-to-many conversation, a bidirectional mobile game, and Instant Messaging (IM) service. Meanwhile, the concept of Internet web browser running in the Personal Computer (PC) can be adopted to various mobile communication devices. The ultimate purpose of the IMS is to allow the users to access the target contents and services with all the types of networks and devices.
Since the call forwarding service of the conventional IMS is targeted for the Circuit Switching (CS) service in the IMS, it provides the number of transfer-target terminal as the number of the recipient terminal. For example, the SIP URI or TEL URI of the transfer target terminal is provided to the calling terminal. In addition to this basic call forwarding service, since the IMS has the architecture suitable for devising a new service by converging various services, it is required to develop new services capable of extending the CS services.